A Distance Based Reliable Cooperative Spectrum Sensing Algorithm in Cognitive Radio

Spectrum sensing is the most critical task in cognitive radio (CR) which needs to be performed very precisely in order to efficiently utilize the underutilized spectrum and to provide sufficient protection to the primary users (PUs). To improve the sensing performance under fading, shadowing and hidden terminal problems more than one CR users collaboratively perform the spectrum sensing called as cooperative spectrum sensing (CSS). In conventional CSS the decision of each CR is fused at fusion center with equal weights. But due to variable distance of each CR from the PU all decisions are not equally reliable and therefore should be assigned different weights according to their reliability. In this paper we propose a new weighting scheme for CSS under Rayleigh faded channel. In proposed weighting scheme, based on the distance of each CR from the PU reliability of CR nodes is determined and correspondingly appropriate weights are assigned to different users. The CSS algorithm using new weighting scheme gives better performance than conventional CSS algorithm.     

Fractal Dual-Mode Open-Loop Quasi-Elliptic Bandpass Filter with Source-Load Coupling

To realize the feature of small size and high selectivity, a microstrip miniature fractal quasi-elliptic bandpass filter (BPF) with two transmission zeros (TZs) near each skirt is investigated in this paper. The TZs are created by source-load coupling between the input and output E-shaped feeding structures. By using a dual-mode Minkowski fractal shorted stub loaded open-loop resonator, the proposed BPF achieved a size reduction of 97.5% compared with the conventional square dual-mode loop BPF. Even mode analysis is adopted to characterize the Minkowski structure. The frequency responses of the current BPF were simulated and measured with good agreement.     

Energy Detection in Hoyt/Gamma Fading Channel with Micro-Diversity Reception

Spectrum sensing is the important function of cognitive radio and energy detection is the most popular technique used for spectrum sensing. Detection of the availability of unused spectrum for the secondary user becomes difficult when the channel is affected by composite multipath/shadowed fading. In this paper, the performance analysis of an Energy Detector in Hoyt/gamma composite fading channel with Maximum Ratio Combining employing micro-diversity is analyzed. Analytical expressions for performance parameters, i.e., the average probability of detection and the average area under the receiver operating characteristics curve are evaluate. The effect of diversity on the performance of energy detector is also studied. Monte-Carlo simulation results have verified the accuracy of the proposed analysis.     

Free-Standing β-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> Thin Diaphragms

Free-standing, very thin, single-crystal β-gallium oxide (β-Ga₂O₃) diaphragms have been constructed and their dynamical mechanical properties characterized by noncontact, noninvasive optical measurements harnessing the multimode nanomechanical resonances of these suspended nanostructures. We synthesized single-crystal β-Ga₂O₃ using low-pressure chemical vapor deposition (LPCVD) on a 3C-SiC epilayer grown on Si substrate at temperature of 950°C for 1.5 h. The synthesized single-crystal nanoflakes had widths of ～ 2 μm to 30 μm and thicknesses of ～ 20 nm to 140 nm, from which we fabricated free-standing circular drumhead β-Ga₂O₃ diaphragms with thicknesses of ～ 23 nm to 73 nm and diameters of ～ 3.2 μm and ～ 5.2 μm using a dry stamp-transfer technique. Based on measurements of multiple flexural-mode mechanical resonances using ultrasensitive laser interferometric detection and performing thermal annealing at 250°C for 1.5 h, we quantified the effects of annealing and adsorption of atmospheric gas molecules on the resonant characteristics of the diaphragms. Furthermore, we studied the effects of structural nonidealities on these free-standing β-Ga₂O₃ nanoscale diaphragms. We present extensive characterization of the mechanical and optical properties of free-standing β-Ga₂O₃ diaphragms, paving the way for realization of resonant transducers using such nanomechanical structures for use in applications including gas sensing and ultraviolet radiation detection.     

Flexible Strain Sensor Based on Layer-by-Layer Self-Assembled Graphene/Polymer Nanocomposite Membrane and Its Sensing Properties

Graphene is a potential building block for next generation electronic devices including field-effect transistors, chemical sensors, and radio frequency switches. Investigations of strain application of graphene-based films have emerged in recent years, but the challenges in synthesis and processing achieving control over its fabrication constitute the main obstacles towards device applications. This work presents an alternative approach, layer-by-layer self-assembly, allowing a controllable fabrication of graphene/polymer film strain sensor on flexible substrates of polyimide with interdigital electrodes. Carboxylated graphene and poly (diallyldimethylammonium chloride) (PDDA) were exploited to form hierarchical nanostructure due to electrostatic action. The morphology and structure of the film were inspected by using scanning electron microscopy, x-ray diffraction and Fourier transform infrared spectroscopy. The strain-sensing properties of the graphene/PDDA film sensor were investigated through tuning micrometer caliper exertion and a PC-assisted piezoresistive measurement system. Experimental result shows that the sensor exhibited not only excellent response and reversibility behavior as a function of deflection, but also good repeatability and acceptable linearity. The strain-sensing mechanism of the proposed sensor was attributed to the electrical resistance change resulted from piezoresistive effect.     

The Nanometer-Sized Eutectic Structure of Si/CrSi<Subscript>2</Subscript> Thermoelectric Materials Fabricated by Rapid Solidification

Nanostructuring is known to be an effective method to improve thermoelectric performance but, generally, it requires complex procedures and much labor. In the present study, self-assembled nanometer-sized composite structures of silicon (Si) and chromium disilicide (CrSi₂) were easily fabricated by the rapid solidification of a melt with a eutectic composition. Ribbon-like samples were obtained with a dominant nanostructure of fine aligned lamellae with a spacing range of 20–35 nm. The thermoelectric power factor of the ribbon was observed to be 1.2 mW/mK² at room temperature and reached 3.0 mW/mK² at 773 K. The thermal conductivity was 65% lower than that of a bulk eutectic sample. The results suggest that this method is promising for fabricating an effective nanostructure for thermoelectric performance.     

Resistance Analysis of Spherical Metal Thin Films Combining Van Der Pauw and Electromechanical Nanoindentation Methods

Although micron-sized metal-coated polymer particles are an important conductive filler material in anisotropic conductive adhesives, the resistance of the particles in an adhesive is not well understood. In this study, a van der Pauw method for spherical thin films is developed and applied to determine the resistivity of 30 μm silver-coated poly(methyl methacrylate) (PMMA) particles. The resistivity is used to interpret resistance contributions in single particle electromechanical nanoindentation measurements, which simulate the compression particles undergo in application. The resistivity was found to be coating thickness dependent for thin films in the range 60–270 nm. Estimation of the resistance of the metal shell using the measured resistivity did not account for the total resistance measured in electromechanical nanoindentation. We therefore deduce a significant contribution of contact resistance at the interfaces of the particle. The contact resistance is both coating thickness and particle deformation dependent.     

Effects of Multiple Stacking Faults on the Electronic and Optical Properties of Armchair MoS$$_{}$$ Nanoribbons: First-Principles Calculations

The electronic and optical properties of armchair MoS\(_{2}\) nanoribbons with multiple stacking faults are investigated using first-principles calculations. It’s interesting that the band gaps approach zero for armchair MoS\(_{2}\) nanoribbons with two and four stacking faults. The gaps of armchair MoS\(_{2}\) nanoribbons with one stacking fault and three stacking faults are converged to 0.46 eV and 0.36 eV, respectively, which is smaller than perfect MoS\(_{2}\) nanoribbons. The partial charge density of armchair MoS\(_{2}\) nanoribbons with two stacking faults shows that the defect levels are originated from stacking faults. The frequency-dependent optical response (dielectric function, absorption, reflectance and electron energy loss spectra) is also presented. The optical results of monolayer MoS\(_{2}\) are in agreement with previous study. The peaks in the imaginary part of perfect armchair MoS\(_{2}\) nanoribbons are about 2.8 eV, 4.0 eV and 5.4 eV and the peaks of the imaginary part of armchair MoS\(_{2}\) nanoribbons with stacking faults are mainly 2.8 eV and 5.4 eV. They are independent of ribbon width. The peaks in electron energy loss spectra move toward larger wavelengths (redshift) due to the introduction of stacking faults.     

Cross-Interaction Between Au/Sn and Cu/Sn Interfacial Reactions

The cross-interaction between Sn/Cu and Sn/Au interfacial reactions in an Au/Sn/Cu sandwich structure was studied. Field-emission electron probe microanalysis (FE-EPMA) revealed that the Cu content in the three Au-Sn phases (AuSn, AuSn₂, and AuSn₄) was very low, less than 1 at.%. This means␣that Cu from the opposite Cu foil did not participate in the interfacial reaction at the Sn/Au interface. On the opposite Sn/Cu side, Au-substituted (Cu,Au)₆Sn₅ formed within the initial 1 min of reflow. With prolonged reflow, the Au content in the Au-substituted (Cu,Au)₆Sn₅ increased and it transformed into a Cu-substituted (Au,Cu)Sn phase with 25 at.% Cu after 1 min of reflow at 250°C. The x-ray diffraction (XRD) pattern confirmed the phase transformation of Au-substituted (Cu,Au)₆Sn₅ to Cu-substituted (Au,Cu)Sn phase. In addition, there was greater Au consumption in the Au/Sn/Cu sandwich joint structure than in the single Au/Sn reaction case, due to some of the Au participating in the opposite Sn/Cu interfacial reaction.     

High-performance C60 and picene thin film field-effect transistors with conducting polymer electrodes in bottom contact structure

C₆₀ and picene thin film field-effect transistors (FETs) in bottom contact structure have been fabricated with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) electrodes for a realization of mechanical flexible organic FETs. The C₆₀ thin film FETs showed n-channel enhancement-type characteristics with the field-effect mobility μ value of 0.41 cm² V^?1 s^?1, while the picene thin film FET showed p-channel enhancement-type characteristics with the μ of 0.61 cm² V^?1 s^?1. The μ values recorded for C₆₀ and picene thin film FETs are comparable to those for C₆₀ and picene thin film FETs with Au electrodes.